A special spreader especially designed for the purpose is used for lifting containers. The spreader comprises a frame and two telescopic beams resting on the frame, the beams performing a telescopic movement, i.e. more exactly, a transfer movement in the direction of the longitudinal axis of the telescopic beams into and out of the spreader frame. Both the telescopic beams generally have two twistlocks. The telescopic beams grip the lifting attachments in the corners of the container with the twistlocks. As there are containers of several different lengths, i.e. the length may vary from 20 feet to 45 feet and even over that, a general-purpose spreader has to have a telescopic movement suitable for each length.
Spreaders are used with various forklifts and rope cranes movable on wheels. In rope cranes, the required energy is fed to the spreader with an electric cable, the spreader being provided with actuators for the necessary movements. The status information of the proximity switches used as accessories for the control system and the control commands from the control system usually travel along the same cable between the spreader and the rope crane.
The operating system for spreaders has traditionally been electro-hydraulic, because the telescopic movements needed in spreaders generally are long linear movements Hydraulic motors have been used as actuators in spreaders for generating a rotating movement, and hydraulic cylinders or chains have been used for generating a linear movement. A hydraulic aggregate and the actuators connected to the hydraulic aggregate require a lot of power because of their efficiency. The sliding surfaces of the telescopic parts in spreaders also require a lot of power for overcoming friction. Further, the hydraulic drive of spreaders usually contains several components, because several movements are needed in the spreaders. Such movements comprise the telescopic movement of the spreaders to the positions of 20′, 40′ and 45′, the turning of four twistlocks, and the use of so-called flippers. The flippers are used for transferring the spreader to the right place above the container.
The spreaders have also been problematic because of several damages and oil leaks soiling harbour terminals. In use, also relatively hard impact stresses are often directed to the spreader, stopping the operation of the spreader for the time of service or repair or possible for the time the entire device is changed. Sea climate again causes fast corrosion damages especially to surfaces, from which use takes away the protective layer of paint, thus causing the spreader to be serviced at more frequent intervals. Such services comprise, for example, the sliding surfaces between the spreader frame and the telescopic beams.
The present spreaders, with which it is possible to grip containers of different lengths by using the telescopic movement of the spreaders, are also relatively heavy in comparison with the load to be lifted, their unloaded weight being about 27%, on average, of the maximum load. A spreader with a large unloaded weight requires a considerable amount of driving energy, which again requires that the lifting gear of the cranes be dimensioned larger.